A number of materials including metals such as aluminum, lead, magnesium, zinc, zirconium, titanium and silicon, for example, can be produced by electrolytic processes. Although individual processes may vary in some respects from one to another, each employs the use of an electrode which must operate in a highly corrosive environment.
An example of such a process for the production of metal is the well-known Hall-Heroult process (hereinafter referred to as the Hall process) for producing aluminum in which alumina dissolved in a molten fluoride salt bath is electrolyzed at temperatures from 900.degree. C. to 1000.degree. C. In the process as generally practiced today, carbon is used as an electrode to reduce the alumina, and the reduction produces molten aluminum, and the carbon is oxidized to primarily form CO.sub.2 which is given off as a gas. Despite the common usage of carbon as electrode material in practicing the Hall process, there are a number of disadvantages to its use.
Since carbon is consumed in relatively large quantities in the Hall process, approximately 420 to 550 kg per ton of aluminum produced, the electrode must be constantly repositioned or replenished to maintain the proper spacing with the cathode in the cell to produce aluminum efficiently. If prebaked electrodes are used, it may be seen that a relatively large facility is needed to produce sufficient electrodes to operate an aluminum smelter. Furthermore, to produce the purity of aluminum required to satisfy primary aluminum standards, the electrode must be relatively pure carbon, and availability and cost of raw materials to make the carbon are of increasing concern to aluminum producers.
Because of the disadvantages inherent in the use of carbon as an electrode, there has been a continuing search for inert or nonconsumable materials that can operate as an electrode with a reasonable degree of electrochemical efficiency and withstand the high temperature and extremely corrosive environment of the molten salt bath. Some inert electrode materials are disclosed in U.S. Pat. Nos. 4,374,050, 4,374,761, 4,399,008, 4,455,211, 4,582,585, 4,584,172, 4,620,905, 5,794,112 and 5,865,980 and U.S. application Ser. No. 09/241,518, filed Oct. 3, 2000, now U.S. Pat. No. 6,126,799 which are assigned to the assignee of this Application and which are incorporated by reference. The material described therein which can be used to form a non-consumable electrode is a cermet.
One problem in the development and use of non-consumable electrodes for producing aluminum by electrolysis has been developing an electrical and mechanical attachment to connect the non-consumable electrode to an electrical source. In a typical operation of a Hall cell using carbon as the electrode, the electrode is formed into a block having a rectangular cross section and a metallic rod or bar is embedded therein by providing a hole in the block, inserting the rod in the hole and filling the void between the rod and the block with molten iron. When the iron solidifies, it shrinks tightly around the bar and away from the hole surfaces of the carbon block, but disengagement is prevented by adapting the block so as to engage the solidified iron. Such an adaptation is providing recesses in the hole sidewall to form a mechanical lock. When the above-described assembly is positioned in a Hall cell having a salt bath which is maintained at approximately 1000.degree. C., the rod, cast iron and carbon in the connection zone rise in temperature from room temperature to approximately 700.degree. to 800.degree. C. The rod and cast iron expand more than the carbon in the connection zone and create a substantially tight and reasonably efficient electrical and mechanical connection.
When using carbon as the electrode body, it is desirable that it be in a block form because it is consumed during the electrolytic process and a large block or mass minimizes the frequency with which electrodes must be replaced. Additionally, the carbon materials are typically better conductors of electricity than are ceramic materials used in inert electrodes. When materials such as cermet are used for electrode bodies, however, such a connection is not necessarily satisfactory for a number of reasons. It is not desirable, for example, to provide a cermet electrode in a large mass or block because, typically, ceramic electrode bodies are more expensive to make than are carbon electrode bodies. Cermet bodies are also subject to cracking and damage. Because of this, and because of the conductivity of cermet, a cermet electrode will typically be formed by disposing a layer of cermet on a conductive material core. To operate, the core must be attached to a current conductor. Because the cermet electrode is not depleted as quickly as a carbon electrode, the former iron rod type connection is not desirable. For an electrode which will be used for an extended period, e.g. 12 months to 24 months, the connection should be adapted for long term use, as well as maintenance operations, and function to maintain the integrity of the ceramic material when subjected to temperature differentials on the order of 1000.degree. C.
Additionally, as shown in U.S. Pat. No. 4,468,300 to Byrne et al., U.S. Pat. No. 4,468,298 to Byrne et al., U.S. Pat. No. 4,457,811 to Byrne, and U.S. Pat. No. 4,450,061 to Rolf prior art electromechanical connections provide an electrode which hangs from the connection causing the electrical connection to be in tension. An electrical connection between a conductive metal and a ceramic or cermet material performs more efficiently while in compression.